In a method for synchronizing a telecommunications network described in International Patent Publication No. WO 96/12360, time marks for special transmission delays as well as a reference signal are incorporated into the transmitted multiplex signal. Each relative delay of the telecommunications network is locally compensated for. This method is suitable for the synchronization of SFN (single frequency network) transmission networks. Only one operating mode is possible therein, however. Only two additional packets are provided in the predefined frame structure.
Transmitting stations in common-wave networks must be synchronized in order to ensure correct operation. There are several concepts for automatic propagation-time compensation in common-wave networks see (ANT Nachrichtentechnische Berichte article, Vol. 6, October 1989, pp. 65-70):
The propagation times on the modulation lines are measured, and the propagation time difference is compensated for, by looping in additional delays, so as to maintain the phase condition; PA1 The data signals to be emitted are transmitted via the modulation line to the transmitters, and are initially buffered there. Emission of the data is then initiated by a trigger signal, synchronously for all transmitters.
In the case of the latter method with buffering, the synchronization signal is critical for maintaining the phase condition. Dissemination of the synchronization signal cannot occur by transmission over the modulation lines, since the problem of different line propagation times exists here as well. All that thus remains is dissemination of the synchronization signal by radio. This requires an additional receiver at each transmitter location for the synchronization signal and possibly an additional separate transmitter for the synchronization signal. Additional hardware is moreover required in each transmitter, in the form of a FIFO memory.
Other conventional methods are based on derivation of the synchronization signal from a time signal transmitter, e.g. DCF77. The synchronicity achieved thereby can be attained, however, only with complex receivers which perform an analysis of the phase modulation of the standard time emission. When working, on the other hand, with the first-method in which the propagation times on the modulation lines are measured, the following procedure is a possibility and involves relatively little outlay: a bridge is connected at one end of the modulation line, advantageously at the transmitter. At the other end of the line, the total propagation time of the outgoing and return lines can then be measured with a suitable instrument.
This process is performed, for example, manually if a fixed compensation for line propagation times is to be accomplished by looping in additional delay devices. This method successful with transmitters which receive their modulation signals via physical four-wire lines. In the case of active transmission paths, however, considerable differences can occur in the propagation times of the outgoing and return lines. To eliminate this disadvantage, one possibility is to emit a modulation signal from the common star point of the transmitter network via one transmitter, and receive it again at the star point using a measurement receiver. It is thereby possible to ascertain the actual propagation time of the signal from the star point via the modulation line and the transmitter back to the star point. This can be done successively for each transmitter, and the propagation time differences ascertained in each case are brought up to a predefined final value by establishing additional delays.
In the "Cityruf" system, automatic propagation-time compensation is performed in a radio paging concentrator, and only the execution of the automatic propagation-time compensation is initiated by the radio paging exchange. From there, test sequences are transmitted via the transmitter network for synchronization purposes, and are received and analyzed in the radio paging concentrator. A microprocessor generates the necessary control telegrams, which trigger all connected transmitters in succession for the duration of the measurement telegrams. At the same time, synchronously with output of the measurement telegram, a time mark is generated. The emitted test sequences are received by a propagation-time receiver. After signal demodulation, the latter supplies an NRZ signal to a cycle and data regeneration circuit. Using the timing cycle that has been recovered in this fashion, the received data signal is read into a pattern recognizer which makes a comparison between the received data stream and the permanently stored content of the test sequence. As soon as a match has been detected, the pattern recognizer outputs a stop pulse.
An article ANT Nachrichtentechnische Berichte, Vol. 6, October 1989, pp. 57-64 discloses a frame structure for a transport data stream in a radio paging system, in which the transmitting stations are controlled by switching telegrams. In addition to data about operating modes and frequency control, a transport data stream of this kind contains synchronization words and data for automatic propagation-time compensation.